A. Field of the Invention
The present invention relates generally to transmission lines, and more particularly to providing an impedance match of an output buffer to a transmission line without significantly increasing the power consumption of the output buffer.
B. Description of the Prior Art
High-speed digital devices are generally interconnected by controlled impedance transmission lines, terminated on both ends with the characteristic impedance of the lines to prevent reflections from distorting the pulse being received. In principle, only one side of the transmission lines needs to be terminated. If it is the receiving end, then no reflections occur and the signal propagates undistorted to the far end load, where it appears as a delayed replica. If only the transmitting side is terminated, then the signal reflects at the receiving end back to the transmitting end, where it terminates. At the receiving end, the pulse remains undistorted, as long as the termination is either open or resistive.
In reality, due to parasitic effects and manufacturing tolerances, the transmission lines and their terminations are far from perfect and reflections in both directions will occur. The best performance is achieved when both terminations are implemented. The drawback to using double termination, however, is increased power consumption. For a given power delivered to the receiving end, the same power must be dissipated in the termination at the transmitting end. This can present a major problem when the output power is large, as is the case in, for example, an array of output buffers or a single, high power load. An example of the latter case is a laser or a modulator in an optical transmitter.
Accordingly, there is a need for providing an impedance match of an output buffer to a transmission line without significantly increasing the power consumption of the output buffer.
A need also exists for providing an impedance match of an output buffer to a transmission line, while still allowing for an adjustable output swing as is required for loads such as laser transmitters and optical modulators.
The present invention provides a system and method for providing an impedance match of an output buffer to a transmission line without significantly increasing the power consumption of the output buffer. In an alternative embodiment of the present invention, a system and method are provided for providing an impedance match of an output buffer to a transmission line, while still allowing for an adjustable output swing as is required for loads such as laser transmitters and optical modulators.